[Metrology] Flatness Of Granite Counter Top

Since it does show contact only at the corners, I'd suspect it does have something to do with being unconstrained now that it is sawed out. If this is a sacrificial piece, grind back the corners a bit and see if the contact spot grows, or disappears. I'm sure it could be lapped flat, but as thin as it is I wouldn't count on it being stable.

I watch the installers put in Italian marble on the nurse stations at the hospital, and it comes in pretty good size slabs, all pretty much cut to print. Where the piece has to be longer than they can practically handle, they shim and play with it until it is very close, and epoxy it all up, then polish to blend. It takes a long time, but the seams are invisible. There is some talent involved. You cannot even see a flaw by eye from a low angle. They have it down really well. But marble isn't as hard as granite. I wonder about the abrasive they use. I'll have to ask or get some next time we do a remodel.
 
here is the test:
calibrated surface plate to Grade A, cannode blue ink , test @72*F
https://plus.google.com/u/0/collection/E8Tpv

before
(snipped pics)
after:

end result: FAIL

the granite counter top is not flat, touching in only 4 corners and hollow in the middle >.001"

your results may vary,
use only as directed, yadda yadda yadda :p
One good test is worth a thousand expert opinions, and perhaps a million hobby-machinist opinions... Thanks, Mike!
 
Will make a really classy bench top! Easy to wipe down.
Much easier to maintain than the diamond plate to which I default, although rounds stuff seldom roll off the diamond plate.

Daryl
MN
 
I seem to recall reading that some colors of granite are more stable than others. Black granite is supposed to be the most stable. I realize that most of us are striving for the holy grail of accuracy but I hope new people, like me, won't get hung up on that. I worked with troubled teens for a while and many of them got hung up on meeting some unattainable goals and it made them feel like a lesser person because of it. The title of this forum is HOBBY-Machinist. I get a little frustrated when I can't get something just right, but I always fall back on the fact that this is supposed to be fun. If I thought that a perfect granite plate was a necessity, I would be severely disappointed. It is a goal, but not a requirement. That money could buy a lot of stock to play with and practice.
 
Last edited:
PaPa Jack,

I couldn't agree any more.

Some of us will strive for the perfect flat rock we can find. Just in our blood. And I've used the concrete floor for a surface plate, too! Not so much for flatness but for a setup we had to do to check concentricity on a 30 foot seal assembly that had to run within .010" T.I.R. end to end and .003" T.I.R. between connections. Each seal assembly had around 12 connections in the 30 foot of length. The real fun was stabbing the seal assembly into the seal bore without it hanging up! We did that vertically. I got way off subject here. Sorry, Ken
 
I have a piece 30x30x1 1/2 thick. It is plenty flat enough for the layout I need to do as a hobbyist. My neighbor that was doing granite work at the time even rounded the corners and polished the edges before he gave it to me.
 
Back
Top